At 109, Alberta Thompson began life in the 19th century, lived every minute of the 20th and, despite some trouble getting around, remains sharp in the 21st.

Until recently, Annie Laurie Williams, 105, climbed up and down the stairs at her Five Points-area home, part of her routine of daily exercise and a diet built largely on fruits and vegetables.

And Dr. Harold Eliason, a retired physician who lives at the Forest at Duke retirement community in Durham, celebrated his 104th birthday in February.

All three centenarians are trendsetters.

About 95,000 Americans are now 100 or older, census estimates show, and their closely watched numbers are predicted to more than quadruple by 2030, reaching 1.15 million by 2050.

How healthy they remain in old age may have a dramatic effect on federal entitlements such as Medicare and Medicaid, health-care experts say. The annual cost for treating elderly and disabled people under these programs is currently $400 billion, Congressional Budget Office numbers show. The vital question: Will people in their 90s and 100s have longer periods of mobility and independence or just more years of disability and dependence?

"If we don't do a better job, this really large group of people who reach advanced old age will be a burden on our health-care system," said Dr. Jack Guralnik, an epidemiologist and gerontologist at the National Institute on Aging in Maryland.

In North Carolina, the cost to Medicare of a chronically ill patient's last two years of life can easily surpass $50,000, according to the 2008 Dartmouth Atlas of Health Care. The healthier the state's older people remain, the more tax dollars will be saved.

"The costs are definitely higher" for chronically ill older people, said Denise K. Houston, a researcher and assistant professor at Wake Forest University's J. Paul Sticht Center on Aging and Rehabilitation. "We are managing diabetes and heart disease, yet still having poor function, which leads to loss of independence earlier."

A 30-year leap in U.S. life expectancy during the last century -- from 47 to 77 -- means that the demographic group known as the "old old" is growing faster than any other. Nearly one out of three women who is 50 today will reach 90, demographers say.

Yet those who have passed the milestone express ambivalence. Eliason, who was born in West Virginia and became a pediatrician, chafed at some of the boundaries aging has placed upon him.

"It's getting kind of tedious; life isn't a bowl of cherries," Eliason said this week. "I'm getting to the point where I couldn't walk a block if I had to."

Eliason remained mobile well into his 11th decade. Only a recent knee injury has limited his walking. He jokingly attributes his good health to a secret fountain of youth, but decades of bowling and golfing were probably more of a factor.

Along those lines, academic studies offer some common sense for people who want to reach the century mark in good shape: Remain active, don't smoke, and eat a sensible diet, typically one low in calories. Houston, from Wake Forest, recently led an academic study showing people with a healthy body weight at age 25 and age 50 were less likely to become disabled in old age.

"The longer you are overweight, the worse off you are," she said. "Obesity puts a lot of stress and strain on the joints and on being able to function independently."

When people reach about 65, doctors can make a fairly accurate prediction of whether they will become disabled by conducting a walking-speed measurement and a low-tech mobility test. Patients have to perform tasks such as standing on one leg with eyes closed for 30 seconds or getting up from a chair, without using hands, as many as 10 times. A low score means an increased risk of disability and death.

Never too late

But even sedentary older people can benefit from more exercise.

"A well-designed program combining aerobic, strength, balance and flexibility exercises can make a difference for those who are at high risk of losing mobility," said Guralnik, the National Institute on Aging researcher, noting that tests for much older people are less rigorous.

Scientists have prolonged lifespan in simple organisms such as yeast by manipulating genes. But researchers say that's a distant prospect for humans, given the medical and ethical issues involved.

As for the substantial risk of developing dementia, studies show that regular exercise reduces that likelihood by 30 percent to 40 percent.

Other factors affecting advanced age aren't as obvious: Really old people typically don't get too stressed out.

"I don't think that much bothers me," said Eliason, the retired pediatrician.

Thompson recalled being so sick in years past that she begged God to come and take her. But she takes comfort in memories of her late son, in seeing her granddaughter, who lives in Raleigh, and in her close relationship with staff members at Aversboro Assisted Living in Garner, where she moved last year."I try to take everything as it comes," Thompson said.

thomas.goldsmith@newsobserver.com or (919) 829-8929

(Sidebar:)

AGING WELL

Two studies earlier this year showed that certain factors may not only lengthen lives, but also provide more late-life years free of disability.

* A study of New England centenarians showed that more than seven in 10 lived with an age-related disease for 15 years or more. Researchers say that means that staying active, or escaping disability, may be more important to long life than remaining disease-free.

* A Boston-based study showed that men at age 70 had a better than even chance of living to 90 if they exercised moderately two to four times a week and did not smoke, have high blood pressure, weigh too much or have diabetes.

On the flip side, if any of those five good-health factors turned negative, it reduced the probability of living to 90 by about 10 percentage points. A man with all five bad markers has a negligible chance of living to 90, the researchers said.

FRIENDS AND A MISSION

It's been said that the only people who want to be 100 are in their late 90s.

For those who'd like to shoot for that level, aging expert Dr. Robert Butler cites three general factors that could help:

* Find purpose: Dote on grandchildren, follow a sports team, or forge an active faith life -- all provide a larger purpose to life that keeps people engaged.

* Foster social networks: Maintain a group of close friends. This is often more difficult for men than women.